1. Field of the Invention
The present invention relates to exposure control devices for use in photographic apparatus, such as still cameras and the like, and more particularly to exposure control devices for automatically establishing exposure in accordance with the level of scene illumination.
2. Description of the Prior Art
In many prior art shutter control devices which include timing circuits for establishing shutter speed, the circuit is energized by a battery contained in the camera. Such mechanisms usually include an electromagnetic transducer adapted to close the shutter after a period of time determined by the timing circuit. Although such arrangements are generally satisfactory, they rely on the availability of sufficient power from the battery. When the battery power is inadequate for operation of such systems, either the shutter operates at a fixed, predetermined speed or it remains open indefinitely, depending on the type of shutter control.
Photographic apparatus such as cameras that employ electric generators for energizing exposure control circuits have been described in the patent literature, such as in U.S. Pat. No. 3,598,030. However, we do not know of any commercial use of such apparatus, which commonly are shown with a spring-driven member to rapidly rotate the generator armature. One possible problem which may have discouraged the use of such generators might be a tendency to cause camera vibration and noise during armature run-down. Also, since the energy conversion of such generators is inefficient, a substantial amount of energy must be stored in the spring, and this is generally done during manual film advance or by preliminary shutter release button movement. The greater the amount of energy needed to power the generator, the more physical effort is demanded from the operator.
In co-assigned British Pat. No. 1,365,560, which issued on Jan. 2, 1975 to D. M. Harvey and G. M. Inglis, a generator-powered electronic shutter mechanism is disclosed which overcomes many of the problems associated with both battery-powered systems and spring-loaded generator armatures. That shutter mechanism is arranged to terminate an exposure in response to a control signal derived from an electronic trigger circuit. The source of energy for the electronic trigger circuit comprises an electric pulse generator which includes a permanent magnet and a coil which lies in the magnetic field. Upon actuation of the shutter release, the coil moves in the field to induce an electromotive force.
While the Harvey et al mechansim operates well for its intended purpose and overcomes many of the problems associated with battery and spring-loaded generator armature-powered exposure control systems, it comprises relatively large masses which are required to move during shutter run-down, which might result in camera movement and/or noisy operation.